To truly unify the Mozilla UI and (as an added bonus) further fuel use of the Mozilla sidebar, Mail/News, Composer, and all other major components of the Mozilla suite should become individual sidebar tabs of the Navigator browser. That is, when I click the Mail/News icon, a process similar to that which opens the Search tab when typing a search term in the Address bar should be triggered. This process should open the sidebar, if it's not open already, and present the Mail/News or Composer tab (or Address Book, or whatever). The Navigator window should then morph into the Mail/News window. Note that there is a precedent for this: when you go to Edit-->Preferences and then go to Navigator-->Internet Search, you can click the Advanced option and used the Advanced search feature. When this Advanced search feature is used, the Navigator windows morphs into a two-paned Search window and the Search tab appears to the left. My argument here is that there is no reason that Mail/News should not appear as bidden in the same manner as the Advanced Search window does: in the same windows as the first, not a different window as is currently the case. Ultimately, I suppose what I'm looking for is a Navigator window that never needs to close depending on the task at hand: the same Navigator windows can be Composer, Mail/News, the Address Book, Calendar, or whatever else. Following this, the "File-->New-->Navigator window menu item could be used to offer up a new window that could wear any hat it needed to: Mail/News, Navigator, Composer, Address Book, ChatZilla, or whatever else, just by integrating the interfaces of each of these into the Navigator sidebar. The window toolbars would need to be able to be changed dynamically, of course.
Adding MPT (curious what he thinks and whether he knows of a dupe).
Until recently, I was certain that bug 21059 was the worst UI idea ever suggested in the entire Bugzilla database. Now that this bug has been filed, I'm not so sure. CCing Hyatt to nuke this from orbit.
I'm tempted to just resolve it as wontfix directly, but I'll let someone else do it.